1. Field of the Invention
This invention relates to a compact-sized stepping motor, and more particulary to improvements in a compact-sized stepping motor capable of being assembled with stator poles thereof disposed at accurate positions.
2. Description of the Prior Art
Stepping motors capable of conducting locating stops at each of several predetermined step phases have been used in various phase control drive portions, thus achieving high precision locating control action over a wide range of industrial applications.
In recent years, a necessity has been voiced to render the aforesaid stepping motor compact in size. For example, when the stepping motor is used in drive mechanisms for a printer device, a head free mechanism of a floppy disc drive and the like, there is a strong demand to make the stepping motor compact in size. More specifically, recently, floppy disc drives and the like, particularly micro-floppy disc drives, in which a magnetic disc is incorporated in a hard plastic cartridge, have been used; and moreover, as the floppy disc itself is rendered compact in size, the disc drive should inevitably be made compact in size. Along with this tendency, it has become a significant question to improve the head feed mechanism, of which is required high precision feed action in close contact with the surface of a magnetic disc, so that it is compact in size.
With the ordinary stepping motor of the prior art, the arrangement of the stator thereof is particularly complicated. Normally, stator coils are wound around every pole or some poles of the stator, the stepping motor as a whole is large-sized owing to the above-described assembling of the coils to the stator, whereby the diameter of rotor is small as compared with the outer size of the stepping motor, thereby presenting such a disadvantage that satisfactory turning torque and locating accuracy cannot be obtained by a small-sized stepping motor.
Furthermore, in a stepping motor, the clearance between a stator pole and a rotor pole is made as small as possible, whereby the torque can be increased. However, in the conventional stepping motor, there has been such a disadvantage that various errors are permitted in assembling of the stator poles, whereby the aforesaid clearance cannot be satisfactorily decreased.
Additionally, in the conventional stepping motor, the center-alignment between the stators and the rotor seriously affects the aforesaid clearance between the poles. However, in the conventional stepping motor, the center-alignment has been carried out by a locating pin provided on a housing or the like, whereby a satisfactory assembling accuracy cannot be obtained, so that an error is caused in the center-alignment between the stators and the rotor, thus presenting various drawbacks.
Particularly, in the stator pole, a plurality of divided stator pole pieces have been assembled into a stator assembly having an entirely circumferential pole at a final stage, whereby errors in the radial direction among the poles are caused. Because of this, unless a comparatively large clearance from the rotor pole is set, such a disadvantage is caused that there occurs interference between the poles, an increase in the variation of the torque and the like.